In recent years, in electrophotographic apparatus such as printing apparatus, it is sought from the viewpoint of energy saving to make a toner more advanced which is readily fixable to a transfer material such as paper at a low temperature. At the same time, with an improvement in resolution of images and in order to make them have image quality close to that of photographs (camera pictures) or photogravures, it is needed to control glossiness of images and also, in the case of color machines, to secure color reproducibility with good and wide-range color mixing. For example, it is needed to obtain images having a high glossiness close to that in image quality of photographs.
For that purpose, it is necessary that binder resins used in toners are made to have a lower glass transition point (Tg) or made to have a lower average molecular weight. If, however, the binder resins used in toners are merely made to have a lower Tg or a lower average molecular weight, such toners may have a low toner strength to cause any component members to become contaminated due to toner melt sticking or exudation of wax at the time of development at a high speed or in the case of a non-magnetic one-component developing system that is feasible for making apparatus compact. Also, in an extreme case, the storage stability of toners may be damaged to make any images obtainable.
More specifically, an attempt to merely improve fixing performance results in damage of developing performance. If on the contrary the giving of any precedence to developing performance may fall into a relationship that the fixing performance is not improved. In particular, from the viewpoint of high definition and high image quality, the toners have a tendency to have a smaller average particle diameter, and this makes it difficult to achieve both the resistance to member contamination caused by toner melt sticking or exudation of wax and the performance of low-temperature fixing. It is required for the toners as an important subject to achieve both the developing stability of toner and the low-temperature fixing performance of toner that are apparently contradictory to each other.
In order to settle such a subject, it has become important to control the internal structure of toner particles, and a proposal is made in which the hardness (micro-compression hardness) of a toner single particle as a unit is defined so as to manifest the durability and fixing performance of the toner single particle as a unit.
Patent Literatures 1 and 2 disclose a capsule toner constituted of a heat-fusible central part (core) composed of a thermoplastic resin having a low glass transition point and an outer part (shell) composed chiefly of a non-crystallizable polyester (i.e., having a core-shell structure). According to these Patent Literatures, it is reported that the relationship between a displacement level and a force where the toner single particle is compressed when a force is applied thereto may be defined within a specific range and this enables simultaneous achievement of the low-temperature fixing performance, anti-offset properties and anti-stress properties of toner. This capsule toner, however, has the structure that cores having a low glass transition point are covered with relatively thick shell layers. Hence, although this is effective in a step of fixing under heating and pressing, it is difficult in a step of fixing under light loading to satisfy the low-temperature fixing performance and high image glossiness that are required for the toner. Also, the core material and the shell material may be so low adherent to each other as to come inferior in the running stability of the toner.
Patent Literature 3 discloses a proposal of a toner characterized in that a force-displacement curve obtained by conducting a micro-compression test of toner particles has a displacement point and that the force at that displacement point is larger than the load the toner receives in a developing assembly. According to this Patent Literature, it is reported that the controlling of the displacement point in the force-displacement curve enables a toner to be obtained which may readily rupture by pressure in the fixing step and nevertheless has good durability in the developing assembly and stable charge characteristics. This toner may satisfy the fixing performance in a usual fixing step. However, where the fixing step is made light-load or high-speed, it is difficult for the toner to enjoy a sufficient low-temperature fixing performance and a high image glossiness.
Patent Literature 4 discloses a proposal of a toner in which a force-displacement curve obtained by conducting a micro-compression test of toner particles has a shoulder, where the maximum value at a middle-point position of the shoulder portion is defined and also the slope of force up to that shoulder is large. As the result, oilless fixing can be performed and high-grade images free of any background staining can be obtained in one-component development where the toner is under constant pressure at the part of toner layer thickness control, as so reported. This toner is effective in the oilless fixing. However, in a usual fixing method, it is difficult for the toner to satisfy a sufficient low-temperature fixing performance and a high image glossiness.
Patent Literature 5 discloses a proposal that, in a force-displacement curve obtained by conducting a micro-compression test of toner particles, a displacement level at the time that a constant force is applied at a constant rate is specified to thereby achieve both the anti-stress properties at the time of development and the performance at the time of transfer and fixing. There, however, is still room for improvement where the fixing step is made more high-speed or high-definition full-color images are to be obtained.